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Mission of sub cut short in search for missing plane

Flight 370 search coordinator Angus Houston cautioned against jumping to conclusions about the oil slick’s source.

Rob Griffith/Associated Press

Flight 370 search coordinator Angus Houston cautioned against jumping to conclusions about the oil slick’s source.

PERTH, Australia — The search area for the missing Malaysian jet has proved to be too deep for a robotic submarine, which was hauled back to the surface of the Indian Ocean less than halfway through its first seabed hunt for wreckage and the all-important black boxes, authorities said on Tuesday.

Search crews sent the Bluefin 21 deep into the Indian Ocean on Monday to begin scouring the seabed for the Malaysia Airlines Boeing 777 after failing for six days to detect any signals believed to be from its black boxes.

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But after only six hours of its planned 16-hour mission to the seabed, the autonomous underwater vehicle, made by Bluefin Robotics Inc. of Quincy, Mass., exceeded its maximum depth limit of 15,000 feet and its built-in safety feature returned it to the surface, the search coordination center said in a statement.

What if anything it might have discovered during the six-hour search was being analyzed. The Bluefin 21 will resume the search Tuesday when weather conditions permit, the statement said.

Search authorities knew that the primary wreckage from Flight 370 was probably lying at the limit of the Bluefin’s dive capabilities. Deeper diving submersibles have been evaluated, but none is yet available in the search area.

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The sub would have been programmed to return to the surface once it exceeded the dive limit, but a safety margin would also have been included to protect the device from damage if it went a bit deeper, said Stefan Williams, a professor of marine robotics at the University of Sydney.

‘‘Maybe some areas where they are doing the survey are a little bit deeper than they are expecting,’’ he said. ‘‘They may not have very reliable prior data for the area, so they have a general idea. But there may be some variability on the sea floor that they also can’t see from the surface.’’

Meanwhile, officials were investigating an oil slick about 3.4 miles from the area where the last underwater sounds were detected, said Angus Houston, the head of a joint agency coordinating the search off Australia’s west coast.

Crews collected an oil sample and sent it to Australia for analysis, a process that will take several days. Houston said it does not appear to be from any of the ships in the area but cautioned against jumping to conclusions about its source.

The Bluefin 21 can create a three-dimensional sonar map of any debris on the ocean floor. But the seabed in the area is covered in silt that could mask part of the plane.

‘‘What they’re going to have to be looking for is contrast between hard objects, like bits of a fuselage, and that silty bottom,’’ Williams said. ‘‘With the types of sonars they are using, if stuff is sitting up on top of the silt, say a wing was there, you could likely see that . . . but small items might sink down into the silt and be covered and then it’s going to be a lot more challenging.’’

The search was shifted to below the surface after crews picked up a series of underwater sounds over the past two weeks that were consistent with signals from an aircraft’s black boxes, which record flight data and cockpit conversations. The devices emit ‘‘pings’’ so they can be more easily found, but their batteries last only about a month and are now believed to be dead.

‘‘Today is day 38 of the search,’’ Houston told a news conference on Monday. ‘‘We haven’t had a single detection in six days, so I guess it’s time to go under water.’’

Flight 370 disappeared March 8 during a flight from Kuala Lumpur, Malaysia, to Beijing with 239 people on board, mostly Chinese.

But Houston warned that while the signals were a promising lead, the public needs to be realistic about the challenges facing search crews in the extremely remote, deep patch of ocean — an area he called ‘‘new to man.’’

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